Basic knowledge of aerosol kinetics is essential for the understanding of many important physiological processes, such as environmental exposure to pathogenic particles, or therapeutic and diagnostic application of aerosol delivered substances. Our major goal is to understand aerosol behavior in the lung periphery. The currently existing mathematical models of aerosol transport and deposition consider the viscous flow reversible and, therefore, theoretically exclude the possibility of convective (flow-induced) mixing in the acinar region. Contrary to these theories, experimental data suggest appreciable convective mixing in the acinus. The recent discovery that viscous flow can be irreversible if the flow exhibits a chaotic structure (Ottino, et al., Nature, 333(6172),419-425, 1988) has opened up a new avenue in fluid mechanics, describing chaotic behavior of fluids as an important factor in mixing. Studying the influence of structural characteristics of the acinar duct, in particular the effects of time-dependent geometric expansion of alveolar walls on flow behavior, we have observed phenomena characteristic of chaotic flow. Based on these observations we hypothesize that complex chaotic flow can occur in the rhythmically expanding-contracting alveolated duct structure during normal breathing and it can result in strong flow-induced (chaotic) mixing. We propose to test this hypothesis with a combination of mathematical analyses - Lagrangian particle tracking and flow visualization in physical alveolus models and with animal lungs. We will quantify the extent of mixing by computing the time evolution of the entropy of tracer particles. This new concept leads to a totally different and quantifiable picture of the previously used ideas of spreading and dispersion. We believe that the results of this proposed study would give us new insight into aerosol kinetics and would help us in understanding important physiological processes.